1. Field of the Invention
The present invention relates to a polling communication system and a polling control method which communicate by successively calling a plurality of sub stations from a main station.
2. Description of the Related Art
FIG. 1A illustrates an example of a general structure of a system in which a plurality of sub stations are connected to a main station via a bus. In FIG. 1A, the system includes a host 100, a main station 101, sub stations 102, data terminal equipments (DTEs) 103, and a bus 104. M, M#1, M#2, . . . represent modems. #1, #2, . . . represent addresses of the sub stations 102. That is, data communication is performed between the data terminal equipments 103 and host 100 by connecting the plurality of sub stations 102 to the main station 101 that is connected to the host 100 through the bus 104.
In such a system using a bus, communication method such as the CSMA/CD (Carrier Sense Multiple Access/Collision Detection) method and a polling/selecting method are employed. The CSMA/CD method is standardized in a LAN (Local Area Network) and the like.
FIG. 1B is a timing diagram for explaining the conventional polling method. As shown in FIG. 1B, for example, a main station inquires sub stations whether or not the sub stations have transmission data by successively specifying an address of each of the sub stations #1, #2, . . . . When a sub station is designated, this designated sub station notifies no-data to the main station when the sub station does not have transmission data, and performs data transmission when the sub station has transmission data. In response to the data transmission from the sub station, the main station provides the sub station with a normal reception notification, and the sub station sends a transmission end notification. Then, the main station inquires a next sub station whether or not the sub station has transmission data by specifying the address of the sub station. Additionally, when a sub station does not respond to the polling, the main station performs the polling according to a next polling address after a predetermined time elapses.
FIG. 1C is a timing diagram for explaining the conventional selecting method. As shown in FIG. 1C, for example, when a main station transmits data by selecting a sub station, the main station inquires the sub station whether or not reception preparation is satisfactory (OK), by adding the address #1 of the sub station, in this case. When the sub station responds to the main station that the preparation is OK, the main station transmits data by adding the address #1 of the sub station. When the sub station normally receives the data, the sub station sends a normal reception notification, and in response to the normal reception notification, the main station sends a transmission end notification.
FIGS. 2A and 2B are flow charts for explaining a conventional polling. FIG. 2A is the flow chart for a main station, and FIG. 2B is the flow chart for a sub station.
First, a description will be given of a process of the main station, with reference to FIG. 2A. In step S11, the main station successively changes an address of a sub station. Then, in step S12, the main station inquires the sub station whether or not the sub station has transmission data, by adding an address of the sub station that is changed this time. In step S13, the main station determines whether or not there is a notification that the sub station has the transmission data. In a case where the sub station has no transmission data (NO in step S13), process returns to step S11. On the other hand, in a case where the sub station responds that the sub station has transmission data (YES in step S13), in step S14, the main station receives data from the sub station. When the main station normally receives the data, in step S15, the main station sends a normal reception notification to the sub station. When the main station receives a transmission end notification from the sub station in step S16, the process returns to step S11.
Next, a description will be given of a process of the sub station, with reference to FIG. 2B. In step S21, each of the sub stations stands by until its address is called. When the sub recognizes that it is called in step S22, in step S23, the sub station determines whether or not there is transmission data. When there is no transmission data (NO in step S23), in step S27, the sub station notifies the main station of having no transmission data. Then, the process returns to step S21. On the other hand, when there is transmission data (YES in step S23), in step S24, the sub station performs data transmission. When the sub station receives a normal reception notification from the main station in step S25, the sub station transmits a transmission end notification to the main station in step S26, and the process returns to step S21.
FIGS. 3A and 3B are flow charts for explaining the conventional selecting method. FIG. 3A is the flow chart for a main station, and FIG. 3B is the flow chart for a sub station.
First, a description will be given of a process of the main station, with reference to FIG. 3A. In step S31, the main station changes an address to the next sub station in sequence. Then, in step S32, the main station inquires the sub station whether or not reception preparation is OK. The main station determines whether or not reception preparation of the sub station is OK in step S33. When the reception preparation is not OK (NO in step S33), the process returns to step S32. On the other, hand, when the reception preparation is OK (YES in step S33), in step S34, the main station transmits data. When the main station receives a normal reception notification from the sub station in step S35, in step 36, the main station transmits a transmission end notification to the sub station.
Next, a description will be given of the process of the sub station, with reference to FIG. 3B. In step S41, the sub station stands by until its address is called. When the sub station recognizes that it is called in step S42, the sub station determines whether or not the reception preparation is OK in step S43. When the reception preparation is not OK (NO in step S43), the sub station sends a reception preparation NG (negative) notification to the main station in step S47, and the process returns to step S41. On the other hand, when the reception preparation is OK (YES in step S43), the sub station receives the data in step S44. Thereafter, in step S45, the sub station sends a normal reception notification to the main station. Then, in step S46, the sub station receives a transmission end notification from the main station, and the process returns to step S41.
The CDMA/CD method generally used in a LAN is standardized by IEEE 802.3, and methods such as 10BASE-T (10 Mbps) and 100BASE-T (100 Mbps) in correspondence with the data transmission rates are known. In such methods, whether or not data communication is currently being performed is determined by carrier detection. A sub station sends data when other sub stations are not performing data communication. In this case, when a plurality of sub stations simultaneously transmit data, transmission collision occurs. When the transmission collision is detected, each of the sub stations randomly sets a waiting time, after which the sub resumes transmission of the data. Accordingly, there is a problem in that, in a high-traffic state, transmission collision occurs frequently and throughput deteriorates.
Further, the polling/selecting method refers to a method in which data communication is performed according to control from the main station. In the selecting method, the main station transmits data by selecting, the sub station. In the polling method, the main station successively specifies the sub stations, and the sub stations transmit data to the main station. In this conventional polling method, even in a case where there are the sub stations that are not online or sub stations in which failure occurs in the plurality of sub stations, all the sub stations are polled in turn. Thus, the sub station that is waiting for data transmission is put in a state of waiting until it is specified. Accordingly, there is a problem in that it is impossible to communicate with a sub station having high traffic proportionally to the traffic.
Additionally, a home electric appliance network is known. The home electric appliance network enables automatic control of various electric home appliances by using interior wiring that provides operating power to the electric appliances in a home or the like as a network for data communication. Generally, the CSMA/CD method is applied to the home electric appliance network. Further, a system is known in which a personal computer, which is connected to an indoor electric power line, is connected to the Internet by a power-line carrier method through an incoming line and low-voltage distribution line. In this case, a modem corresponding to the main station is provided as a transformer that converts high voltage of 6600 V to low voltage of 100 V or 200 V. In addition, a modem corresponding to the sub station is provided indoors. Thus, a system is structured in which data communication is performed by using the low voltage distribution line and incoming line as transmission channels of a power-line carrier system. For that purpose, the indoor electric power line is shared. Thus, there is a problem in that mutual interference occurs.